The development of multi-payload communication satellites has given rise to a need for ground terminals with simultaneous frequency reuse capability at more than one frequency band. This, in turn, requires a feed system that is capable of operating in a plurality of different and widely separated bands. For satellite communication applications, the feed should preferably operate simultaneously at three separate receive bands with wide bandwidths; present satellite telecommunications systems generally downlink signals in the 3.40-4.20 GHz band (known as the C-band); the 7.25-7.75 GHz band (X-band); and the 10.7-12.2 GHz band (Ku-band). At each band, the polarization should be independently and remotely selectable, so that either horizontal and vertical linearly polarized signals or right-handed and left-handed circularly polarized signals can be received. The feed should also provide polarization isolation of better than 30.7 dB (equivalent to an axial ratio of 0.5 dB) between the orthogonal signals when installed on the host reflector antenna. The triband feed system must also exhibit a good match at the output ports, as well as low insertion loss.
Several techniques are currently available for implementing the multiband requirement. The simplest way to provide simultaneous multifrequency operation is to use three separate antennas. Such an approach has some advantages over any solution involving a single antenna; it provides the best electrical performance, because each feed is optimally designed for its own band and therefore has low insertion loss, and it enables a smaller antenna size to be used for an equivalent G/T performance. In addition, the design of the feed configuration is obviously much simpler. However, this method has the disadvantage of requiring the installation, operation and maintenance of three antennas, at a significantly higher cost than for a single antenna.
A second method of providing a multiband band involves the use of three separate movable feeds, one feed for each band, with a single reflector antenna. However, if three different feeds are positioned around the antenna focal point, then two of the secondary beams are squinted with respect to the main axis, leading to substantial loss in gain and cross-polarization isolation. In order to obtain zero beam squint, each feed must be moved into the focal position; such an arrangement seriously impairs the ability of the antenna to operate nearly simultaneously at the three different bands.
A third multibanding technique involves the use of a frequency selective subreflector in a dual reflector configuration, in order to separate one of the frequency bands; thus, for example, in the above triband system, the feed behind the subreflector could operate at the Ku-band while the feed at the focus would operate at both the C-band and the X-band. The subreflector would reflect the C-band and X-band with low loss, as if it were a metallic surface, and would act as a high pass filter for the Ku-band. This would separate the three bands into two groups, thereby somewhat simplifying the design problems of a multiplexed feed. However, implementation of this method requires a major modification to existing reflector antennas and is, as such, relatively complex and expensive. In addition, because of the strict dimensional constraints on the frequency selective subreflector, the subreflector must be used in a controlled environment (such as a radome) and is unsuitable for use with an unprotected antenna, where snow and ice on the surface of the subreflector might affect its proper operation. Also, fabrication of a shaped subreflector involves a series of precise and therefore expensive manufacturing steps. In addition, the resulting performance degradation at the Ku-band, because of the absence, due to the shaping process, of a distinct focal point therefore, is greater than with other methods.
Accordingly, it is seen that a multiplexed feed system has a number of advantages over the other methods previously used. Only a single antenna structure is required for operational coverage of all three frequency bands, and in modifying standard reflector antenna configurations or retrofitting existing stations, the reflector antenna surface need not be changed, only the feed being replaced.
Several different devices for transmitting multiplexed microwave signals are known to persons skilled in the art. In one such device, the zero-dB coupler, the signals are coupled from a center or main waveguide through four distributed series of longitudinal slots, each slot having two planes of symmetry, to a set of auxiliary rectangular waveguides. Each pair of diametrically opposed slots couples one polarization from the main guide. Opposite pairs of rectangular waveguides are fed into "Magic T" junctions, which in turn group the orthogonally polarized signals into a polarization combiner, wherein the signals can be rotated and/or converted to circular polarization. However, practical implementations of this configuration have high coupling losses, and so the zero-dB coupler is used mainly to provide tracking functions or communications in transmission bands where losses are not critical. The complexity of the device also leads to high manufacturing costs.
In another such device, a co-axial guide, a plurality of concentric guides are used to multiplex and separate the plurality of frequency bands. However, this device is inherently lossy, and the abrupt junctions therein generate higher order modes, degrading the cross-polarization performance.
Yet another means of multiplexing and propagating the signals makes use of a dielectric rod. To ensure that the high frequency signals (which is carried by means of a surface wave mode, rather than the waveguide mode of the lower frequencies) is bound closely to the surface, the relative propagation constant between surface wave and free space must be in a prescribed range. However, for signals in the frequencies of interest, the resulting diameter of the rod is such as to perturb the lower band (that is, the 4 and 7 GHz) signals. The dielectric rod is therefore more appropriate for use with the propagation of extremely high frequency signals, an application in which the diameter of the dielectric rod can be made suitably small.
In another technique, the polarization diplexing concept, the multiplexed signals are separated by polarization through a wideband orthomode junction; the horizontal polarized signal is coupled out through a side port while the vertically polarized signal propagates directly through, separation being achieved by means of metallic plates. Each of the signals is then divided into the plurality of bands by a multiplexer. Difficulties arise with the design of the wideband orthomode junction and multiplexer, as well as with amplitude and phase matching of the devices for the orthogonal paths. The use of polarization diplexing is thus usually limited to applications where the signals are restricted to two bands and linear polarization.
The branch filtering concept makes use of multiplexing the signals in the different band frequencies and then using junction devices, spaced along the common tapered waveguide which propagates the signals, to couple the signals of the different band frequencies in and out of the waveguide. This promising approach, subsequently developed for use with the present invention, was applied to the design of a dual frequency band antenna feed, as described in an article by I. Sato, S. Tamagawa, I. Mori, R. Kuzuya, and A. Abe entitled "Dual Frequency Band Antenna Feed Design", published in the Proceedings of the 1985 Europen Microwave Conference, held at Paris, at pp. 445-450 thereof.
A number of patents have been addressed to microwave signal processing and transmission. Canadian Pat. No. 1,190,317 discloses a primary source for a ground-based space communications antenna operating with utilization of the same frequency band in two orthogonal polarizations. In one embodiment, an orthomode junction coupled to a corrugated horn has extending therefrom two channels, an emission channel and a reception channel. The reception channel comprises a higher mode coupler, a 180.degree. degree polarizer, a 90.degree. polarizer, and on orthomode transducer whose polarization accesses are coupled to the reception accesses of the primary source through two rejection filters; the emission channel comprises an orthomode transducer coupled in series to a 90.degree. polarizer and a 180.degree. polarizer. In a second disclosed embodiment, the 90.degree. polarizers are not placed in the emission and reception channels, but rather between the horn and the orthomode junction.
U.S. Pat. No. 3,978,434 discloses a system separating filter for separating two signals, each of which consists of a doubly polarized frequency band, the bands being of different frequency. The filter has three series connected doubly polarizable sections, the first waveguide section having an inner cross-section of such dimension that both frequency bands with their respective double polarizations can exist therein, the second waveguide section serving as a transition between the first and third waveguide sections, and the third waveguide section having an inner cross-section of such dimensions that at least the second frequency band with its double polarization can exist there. A pair of coupling means, each associated with a respective one of the two polarization directions, are provided for decoupling and passing the first frequency band while effecting a total reflection of the second frequency band. A polarization filter, connected to the third waveguide section in which only the second frequency band propagates, provides separate signals corresponding to the two polarizations of the second frequency band at its outputs.
U.S. Pat. No. 4,504,805 discloses a combiner for transmitting and receiving co-polarized microwave signals in a selected propagation mode in at least two different frequency bands. The combiner comprises a main waveguide dimensioned to simultaneously propagate signals in the different frequency bands, first and second junctions spaced along the length of the main waveguide for coupling the signals in and out of the main waveguide, and filtering means within the main waveguide for passing signals in the second frequency band past the first junction.